I own both and have played a good number of games on both. There is an incredibly small difference in the Wii U's favor, and it is almost entirely a matter of color quality.
If you had both a PS3 and Xbox 360 and played the same games on both you'll have some idea what I'm talking about. PS3 games almost always had a more washed out color spectrum. Nothing debilitating, just not as color rich as the X360 version and no amount of TV tuning would really fix it. The Wii U > Wii difference is similar, but even less dramatic.
This is my biggest problem with the Wii. I have mine hooked up to a Sony Wega CRT (that can do 720p/1080i) and it looks pretty good, but even there the colors look a bit more washed compared to my Xbox, PS2, etc.. Nintendo skimped on the video out hardware, just like they skimped on the audio out hardware (did the same with the Wii U, but technological advancement alone made both sets of hardware better than the Wii).
Honestly, I can't imagine hooking my Wii up to an HDTV. The image quality is better on my 480 supporting CRT, the motion controls feel more responsive due to less input lag, etc. etc.. It's a good experience still on a properly calibrated CRT and only goes downhill the more modern set you plug it into.